Background: The purpose of this study was to effect of ambient pH and different two chewing cycles of wear on restorative nanohybrid- and microhybrid-filled composite material in intraoral tribology. Methods: All specimens were stored in the artificial saliva for 7 days before the chewing tests. Then, composite specimens were subjected to chewing simulation using a computer-controlled chewing simulator (F = 50N [vertical movement 2 mm and horizontal movement 0.7 mm] 120.000 and 240.000 chewing cycles, 1.2 Hz frequency, and 37°C ambient temperature), immersed artificial saliva (about pH = 5.7), and citric acid solution (about pH = 3.2). Steatite balls were used as antagonists for every experiment chewing test cycle (d = 6 mm). Specimen's microhardness values determined with Vickers hardness method before chewing tests. Mean volume loss values were determined using noncontact three-dimensional (3D) laser scanning device. Furthermore, two-dimensional and 3D analysis of the mean volume loss wear surface of the composite material was performed both immersed artificial saliva and citric acid. Mean values and standard deviations were calculated and statistical analysis was performed using one-way analysis of variance. In addition, scanning electron microscopy analysis was performed to examine surface wear tracks. Results: The surface morphology of both composite materials was severely damaged in immersed citric acid solution. This can be explained by the more damage to the organic matrix structure of the composite material in citric acid solution than immersed artificial saliva. Conclusion: The results obtained in this study will help to understand the effect of the pH value of the third abrasive surface on the composite material in the three-body wear mechanism.

Note: This study has been extended and revised after been presented in International Conference on Advanced Engineering Technologies, 21.23 September 2017, Bayburt, TURKEY. It was previously published as full text report in the Book
of the conference.

Introduction

The wet and warm nature of the oral environment affects the mechanical and esthetic properties of the tooth and dental composite materials. The mouth area with a mean pH value of 7, and 100% humidity is continuously variable depending on the received food.[1] This structure, which constantly changes in the mouth environment, significantly affects the mechanical and tribological properties of composite restorative materials. The filler content and matrix structure of the composite material are affected differently from the changing pH value in the intraoral tribology. Today, composite materials have a good clinical life although surface roughness and microcracks have been reported in many studies.[1] Damage due to poor mechanical properties and wear will cause the moisture (pH) in the oral environment to cause impact on the composite and bonding materials, degradation, and erosion.[2],[3],[4] Moisture in the mouth, hydrolysis, or enzymatic hydrolysis may cause chemical degradation of composite resin materials.[5] The susceptibility of dental composite resin materials to chemical degradation depends on the different monomers and the degree of crosslinking that make up the resin matrix.[4],[6] Furthermore, the filler type and volume fraction affect the behavior of water absorption and solubility.[1],[7] For this reason, the presence of water is very important for the degradation of composite resin materials.[1] The pH value in the mouth changes continuously during the transport of food.[1] During chewing, food and composite materials are repeatedly chewing force, while food particles in the environment are constantly transported in the mouth.[8] It has been shown that a lower pH adversely affects wear resistance of composite resin materials.[1] Restorative composite materials and teeth are subjected to high physical and chemical conditions in the laboratory environment.[1] These tests are divided into in vitro and in vivo tests. Evaluation of in vivo test assays of composite materials is time-consuming and expensive due to ethical reasons.[1] Because of these reasons, there is a tendency toward in vitro studies in the literature.

In general terms, wear can be defined as the gradual loss of volume with the interaction of two surfaces in contact with each other. There are four basic mechanisms of wear in intraoral tribology. These types, two-body abrasion (occlusal contact area or attrition), three-body abrasion (contact-free area), fatigue wear, and corrosive wear. Two-body wear mechanism concerns the direct contact of two surfaces with each other without the existence of another body. Three-body wear mechanism occurs with the formation of a third abrasive surface between the two surfaces. This third abrasive surface is the food taken in the oral tribology. The behavior of three wear is the continuous movement of food particles between the composite material and the antagonist material under chewing motion. Fatigue wear occurs due to repeated loads on the composite material. As a result of these loads, microcracks occur in the internal structure of the composite material. Chemical reasons and acids originating from food can cause corrosive effects on the surface layer. This acid layer can easily be carried in the mouth with the antagonist. These types of wear can occur alone or in combination.[9] The constant variable structure of intraoral tribology directly affects the wear parameter. Intraoral tribology wear is a process depending on many parameters. The restorative material type, oral environment, mechanical loading, and antagonist structure can play an important role. For intraoral tribology, the mechanical loading is constantly changing due to eating motion. This continuous changing mechanical loading carries out a variable mechanical stress on the dental composite materials. The mechanism of wear that occurs in intraoral tribology is affected by changing mechanical loads. The chemical and mechanical properties of the third abrasive surface in the three-body wear mechanism type of wear can positively or negatively affect wear.[10] Many in vitro experimental systems have been developed for 40 years.[11],[12] However, none of these test systems can simulate intraoral tribology due to the complex structure of the intraoral tribology. In addition to this, none of the many dental wear testing machines have been included in an international standard. However, in 2001, the standard parameters were established for two-body wear and three-body testing with a technical ISO specification.[12] In this specification, the abilities of the chewing simulator and the experimental parameters are determined. The chewing simulator used in this study has the capability of three different chewing simulations in the ISO 2001 technical specification. The purpose of this study was to effect of ambient pH and different two chewing cycles of wear behaviour on composite material through chewing simulation. The results obtained in this study will help to understand the effect of the pH value of the third abrasive surface on the composite material in the three-body wear mechanism. Effect of various mechanical loading of two-body wear on different two dental composite materials.

Materials and Methods

The chemical and mechanical properties of the nanohybrid-filled Charisma and microhybrid-filled G-aenial composite materials tested in this study are shown in [Table 1] (information provided by material manufacturers). A total of 40 specimens (2 mm diameter and 2 mm height) consisting of 10 specimens for each material were prepared following the manufacturer's instructions.

For investigation to dental materials, the chewing simulator device capable of simulating the artificial mouth environment was designed and produced by the research group. [Figure 1] systematically represents the dual-axis movement of the chewing simulator. Chewing simulator performed load 50N in the vertical axis and 0.7 mm horizontal axial movement when the antagonist material touches the specimens (detection was performed with magnetic sensor) When the loading effect on the vertical axis on the specimen is unloading, specimen returns to the starting point again [Figure 1]. Thus, during the chewing tests, wear was formed in the same region of the material surface. A cycle of chewing with chewing simulation represents single human masticatory cycle.[13]

Data were analyzed using statistical software (SPSS Statics 20.0 for Windows 64 bit; SPSS Inc., Chicago, USA; license by Ataturk University). Means and standard deviations of volume loss and Vickers hardness were calculated and analyzed using one-way analysis of variance. The Games–Howell test was used for Post hoc analysis because of Levene's test showed significant differences in the variance of the groups. Regression analysis was performed to investigate the relation between Vickers hardness, filler content, and volume loss. The level of significance was set to α = 0.05.

Results and Discussion

[Table 2] shows the Vickers hardness value and mean volume loss (in artificial saliva and in citric acid, respectively) of both composite materials after 120,000 and 240,000 chewing cycles. The results obtained in this study show that the mean volume loss on the surface of the composite material increases in the citric acid environment compared to artificial saliva environment. This can be thought of as the result of the more corrosive effect of the in citric acid environment than artificial saliva environment. [Figure 2] shows during the chewing tests, wear was formed in the same region of the material surface.

[Figure 3] and [Figure 4] depicted the mean volume loss of both composite materials in citric acid environment after 120,000 chewing test (Charisma and G-aenial, respectively). [Figure 3] and [Figure 4] shows that the surface of the G-aenial composite material was subjected to more damage compared to Charisma composite material. This can be explained by the fact that the matrix structure of G-aenial composite material is more affected with hydrolytic degradation.

The improvements in the component structure of dental composite materials are called micro-and nano-structures as chemical structures. These materials are called micro- or nano-hybrid resin composites depending on the size and content of micro- or nano-particle.[14] In addition, such resin composites are called universal. It is difficult to distinguish these structures in commercial composite materials because of both the microstructure and mechanical properties tend to be similar.[2] The tested dental composite materials in this study are available at the market and widely used in dental treatment. It has been reported in the literature that many chewing simulator devices can simulate both two-body wear and three-body wear mechanisms.[3],[4],[5],[6],[7] Both of them are accepted simulator models for in vitro wear testing. When the two simulator models are compared; in the case of type two-body wear mechanism, wear occurs with direct contact of between test specimens and antagonist specimens, while in type three-body wear occurs with abrasive slurry (for example, poppy seed or polymethyl methacrylate as the third body) between the test specimens and antagonistic specimens.

The intraoral tribology wear mechanism varies depending on many parameters; oral medium, type of composite material, amount of applied vertical force, horizontal wear distance, number of chewing cycles, and antagonist structure. The amount and direction of force applied to the composite material during chewing tests must be similar to in vivo studies (in the oral environment). In the literature, studies have shown that dental and composite materials are exposed to variable loads between 20 N and 120 N in the vertical axis.[8] It is thought that the continuous load change on the teeth and composite material is caused by the breakage of food during chewing. In this study, the average vertical load of 50N was considered as the average chewing force. In addition, thermal change temperature (5°C–55°C), contact time (400–600 ms), loading frequency and number, thermal change temperature, and waiting time are important factors affecting wear of composite materials. It has been reported in the literature that the number of mechanical cycles varies from 50,000 to 1,200,000 in in vitro chewing tests.[9]In vivo study, it has been reported that the average number of chewing varies between 300 and 700.[9] The number of selected 120,000 and 240,000 chewing cycles in this study is clinically correspond to approximately 1 and 2 years in vivo study, respectively. The chewing simulator was programmed to perform a 2-mm vertical movement and a 0.7-mm lateral movement during the experiment and the frequency of load cycle at 1.2 Hz. In the literature, experimental data on changing ambient pH environment of composite materials are very limited. Therefore, in this study purpose investigate effect of ambient pH environment on wear behaviour of composite materials through chewing simulation. Composite materials tested in this study were kept at 37°C for 7 days before the chewing test. The water absorption parameter significantly influences the mechanical properties of dental composite materials. Previous studies have reported that the two-part resistant abrasion resistance and hardness significantly increased in the 7-day stored distilled water compared to the 24-h exposure of the samples only to distilled water.[10],[11],[12] According to Chadwick et al.,[13] no significant differences in wear after 1 week and 1 year of water storage of composite resins was found.[13] Therefore, it can be assumed that the composite material is completely saturated with 1 week distill water stored.

The composite materials tested in this study exhibited different wear resistances in both artificial saliva and citric acid environment. The mean volume loss values in all samples increased in the citric acidic environment. The results obtained in this study can be attributed to the fact that the acidic environment in the mouth more affects wear resistance of composite materials compared to artificial saliva environment. [Figure 5] and [Figure 6] show that in the acidic environment, deep microcracks occur and progress on the surface of the composite material. It is also seen that Charisma composite material has a more uniform wear tracks [Figure 3] than G-aenial composite material [Figure 4]. Moreover, small voids are seen on the entire surface of this material along the edges. This is because mechanical impacts of vertical loading cause plastic deformation in the composite material. This can be explained as the sliding motion of the particles that are snapped of the material wears surface. It has been reported in the literature that filler particles of the composite material have a significant effect on both hardness and two-body wear resistance.[14] In this study, both materials exhibited different wear behaviors because of it has been different filler type, filler volume/weight, and filler contents. These differences may be related to the distribution of the filler particles, monomer properties, and the bond between the matrix and filler. Condon and Ferracane Emphasized that the effect of filler volume on wear resistance is linear.[15] However, there was no correlation between two-body wear resistance and filler volume in the regression analysis in this study. Hahnel et al. did not find a linear relationship between two-body wear resistance and filler volume in their work.[14] The scanning electron microscopy (SEM) evaluation of both composite materials tested in this study indicated that composite materials exhibit different wear behaviors after chewing tests. The reason for this difference is that some of the filler particles contained in the monomer structure of the composite materials and their distribution may be related to the bond between the matrix and filler materials with the conversion grade, filler material, and matrix properties.

Within the limitations of the present study, it can be concluded that nanohybrid composite material Charisma exhibit better wear resistance (both in artificial saliva and in citric acid) than microhybrid G-aenial composite material. This can be explained as Charisma could most likely be attributed to the unique polymer structure. SEM analysis after 240,000 mastication cycles in citric acidic environment shows microcracks and pits on the surface of the material. These microcracks can be the continuation of cracks that occur subsurface of material. This can be suggested as an indication of fatigue wear.